Electrophoresis is based on the principle that charged particles suspended between opposite poles and in an electric field migrate toward the pole possessing the charge opposite that of the particle. The extent of migration is an indication of the composition of the particles. Electrophoretic separation is often used to separate DNA or RNA fragments generated as part of nucleic acid sequencing procedures.
A conventional apparatus for performing electrophoresis is described in detail in U.S. Pat. No. 4,773,984 to Flesher et al., the disclosure of which is incorporated herein by reference. A typical apparatus includes a gel mold composed of two flat glass plates separated by thin spacers placed at opposite edges. A polyacrylamide gel slab is cast between these plates. The electrophoretic separation will be carried out in this gel slab. Some apparatus are designed to orient the gel vertically; others are designed to orient the gel horizontally. A vertical orientation has generally been found to be preferred for the electrophoresis of nucleic acids in such applications as nucleic acid sequencing. A support platform having means for securing the gel mold to the support platform supports the gel mold.
The apparatus also has two reservoirs for containing a buffer solution, one installed toward the upper end of the support platform and a second installed toward the lower end of the support platform. An electrode is installed in each reservoir to apply a voltage to the buffer solution in each reservoir. These electrodes are typically made from platinum. However, platinum electrodes are costly and must be replaced frequently because the electrodes become brittle after several uses. Thus, what is needed is an electrode that would be a sufficient conductor of electricity and be less fragile and less costly than conventional platinum electrodes.
Placement of the gel mold against the support platform situates the gel slab so that when buffer solution is added to each of the reservoirs, an effective electrical contact is established between the buffer solution in one reservoir and the buffer solution in the other reservoir through the gel slab. Thus, most of the voltage differential between the electrodes occurs within the gel slab. Conventional electrophoretic separation is performed by applying between 1500 and 1800 volts, approximately 60 Watts, of electricity to the buffer solution and the gel slab. This involves a pre-run time, for heating the unit, of twenty to thirty minutes and a nominal run time, for completing electrophoresis, of eighty to eighty-five minutes.
When electrophoretic separation is performed, substantial heat is generated in the gel slab. Typically, the heat is removed by placing a large conducting plate in contact with the gel mold. Often this conducting plate is incorporated as part of the support platform against which the gel mold is placed. Means are provided for securing the gel mold against the conducting plate once it has been inserted into the support platform.
The process of performing electrophoretic separation includes: casting a gel slab in the gel mold, preheating the gel mold in the electrophoresis apparatus, inserting samples into the gel slab, applying electricity across the gel slab to perform electrophoretic separation, and removing the gel mold from the apparatus for developing the gel slab. This process is lengthy, often taking two full days to complete testing of a single gel slab. One method available to accelerate the preheating and testing stages of the process is to increase the voltage across the gel slab. However, an increase in voltage causes excess heat to build up in the gel slab, causing hot spots. The resulting variation in temperature across the gel slab can cause the samples to separate through the gel at different rates, thereby causing a "smile" or "frown" shape in the resulting bands. These distortions make comparison of the resulting bands difficult. Further, the hot spots present a danger to the samples because severe temperature fluctuations may destroy a sample.
The conventional conductive plate is insufficient to dissipate heat from the gel slab when the voltage is significantly increased. Thus, a cooling system is needed that will dissipate heat from the gel slab during accelerated electrophoretic separation and that will maintain the gel at a constant and evenly distributed temperature across the gel slab.